It depends on the students age a suppose. I never tell them that because it
is one of the signs when they reach that stage in their developement that
they make a line on the bottom of the page for the earth and often one on
top of the paper to represent the sky. It is how children at that stage of
development see this and it would be wrong to try to make them see it our
way. Anyway does the sky actually touch the ground? or is it like you
said:<the sky actually "appears" to go all the way down and touch the
ground>. I think the important thing is to tell the student how beautiful
it is and slowly but surely she will see it the other way. The same goes
for the ground line. By using artwork in your teaching (DBAE) the students
learn very soon to apply many different colours to the sky and not just do
a strip of blue.